FIG. 1 is a perspective view showing an optical path of a conventional optical recording and reproducing apparatus of the type disclosed in, e.g., JP-A-61-210541. In the diagram reference numeral 1 denotes a light source which may be a semiconductor laser; reference numeral 2 denotes a collimator lens; 3 is a light flux shaping prism for shaping parallel light fluxes; 4 a polarizing beam splitter; 5 a reflecting mirror; and 6 a 1/4 wavelength plate. In this embodiment, the elements 3 to 6 are integrally attached to each other. Reference numeral 7 denotes an objective lens and 8 indicates an information carrier. Only a part of the information carrier 8 is illustrated in the diagram. Reference numeral 8a denotes a recording film formed in the information carrier 8a; 9 is a convex lens; 10 a half prism; 11 a two-split type photo- detector for detecting any error in the track position; 12 a knife edge for shutting out a part of the light fluxes; 13 is a two-split type photodetector for detecting any error in the focal point; 14 parallel light fluxes which are transmitted from the collimator lens 2; 15 light fluxes transmit- ted from the light flux shaping prism 3; 16 the rotational center of the information carrier 8; and 17 reflected light fluxes transmitted from the polarizing beam splitter 4.
The operation will now be explained. The divergent bundle of rays emitted from the semiconductor laser 1 are converted into almost parallel light fluxes 14 by the collimator lens 2. The parallel light fluxes 14 enter the prism 3 and are magnified in one direction which is perpendicular to the optical axis of the parallel light fluxes 14 and become the light fluxes 15.
The above constitution will now be further described in detail with reference to FIG. 2. In the diagram, the light flux shaping prism 3 magnifies only those light fluxes which are travelling in the direction parallel to the paper surface relative to FIG. 2. Namely, the parallel light fluxes 14 transmitted from the collimator lens 2 are magnified since the light fluxes corresponding to light flux diameter h in the direction parallel to the paper surface pass through the prism 3, whereby the light fluxes 15 having a light flux diameter H are obtained. The light fluxes 15 pass through the polarizing beam splitter 4, are reflected by the reflecting mirror 5, and pass through the 1/4 wavelength plate 6. Then the light fluxes are focused by the objective lens 7 to form very small light spot on the recording film 8a of the information carrier 8. The light reflected from the information carrier 8 passes through the objective lens 7 and 1/4 wavelength plate 6 and is reflected by the reflecting mirror 5. Then the reflected light is reflected by the polarizing beam splitter 4 and becomes reflected light fluxes 17. The reflected light fluxes 17 are converged by the convex lens 9. Almost half of the light amount of the light fluxes converged by the convex lens 9 is transmitted through the half prism 10 and enters the two-split type photodetector 11 to detect any error in the track position. The photodetector 11 is arranged at a position away from the focal point of the convex lens 9. The other half of the light amount of the light fluxes converged by the convex lens 9 is reflected by the half prism 10. Almost half of these light fluxes are shut off by the knife edge 12 and then they enter the two-split type photodetector 13 to detect any error in the focal point.
The photodetector 13 is disposed on the focal point of the convex lens 9.
A track position error signal is obtained from the difference between output signals of the photodetector 11. A focal point error signal is derived from the difference between output signals of the photodetector 13. These error signals are input to a control circuit (not shown). An output of the control circuit is input to an actuator (not shown) to drive the objective lens 7. The actuator drives the objective lens 7 in the direction of the focal point, i.e., in the direction of an optical axis (indicated by an arrow F in FIG. 1) and in the radial direction (indicated by an arrow T in FIG. 1) of the information carrier 8. When the information carrier 8 rotates around the rotational center 16, the tracking control which allows the light spot to follow the eccentricity of the information carrier 8 and the focusing control which allows the light spot to follow the surface oscillation of the information carrier 8 are executed. On the other hand, the reproduction signal which is designed to reproduce the information recorded on the information carrier 8 can be obtained from the sum of the output signals of the photodetector 11.
As mentioned above, according to the conventional optical recording and reproducing apparatus, the parallel light fluxes emitted from the collimator lens are magnified in one direction by the light flux shaping prism. In this case, if the relative position of the semiconductor laser and collimator lens is caused to deviate due to an initial adjustment error or the like, the light fluxes which enter the light flux shaping prism may become convergent light fluxes which are slightly converged or divergent light fluxes which slightly diverge, though they are inherently designed to become parallel light fluxes.
As is well known (for instance, refer to JP-A-62-12934), if the light fluxes which enter the light flux shaping prism slightly change to become convergent light fluxes or divergent light fluxes, astigmatism occurs when they pass through the light flux shaping prism, and an astigmatic difference therefore occurs in the light spot which is focused by the objective lens. Thus, there are adverse consequences such as deterioration of the recording characteristics or the like.
Although the allowable amount of any error representative of the relative position of the semiconductor laser and collimator lens varies in accordance with the particular degree of magnification of the optical system concerned, the degree of magnification of the light fluxes, or the like, it must generally be set at about 1 .mu.m or less. In assembling the conventional apparatus, therefore, in order to realize an adjustment accuracy of 1 .mu.m, the adjusting mechanism or the fixing method used after adjustment becomes complicated.
On the other hand, the distance between the semiconductor laser 1 and the collimator lens 2 generally shows a dimension of about 10 mm due to disturbances that occur during operation of the apparatus. Therefore, in the case of using an ordinary material, e.g., aluminum or the like, the distance between the semiconductor laser 1 and the collimator lens 2 may change by 5 to 6 .mu.m in response to a change in temperature of 25.degree. C. Accordingly, it is necessary to take some countermeasures to suppress the potential change in this distance to about 1 .mu.m or less.
According to JP-A-62-12934 which represents one example of the countermeasures that can be taken, an actuator may be attached to a collimator lens and, by driving the actuator, the collimator lens is moved in the direction of the optical axis. This conventional system will be explained with reference to FIG. 3. In the diagram, reference numeral 200 denotes an actuator adapted to move the collimator lens in the optical axis direction. The actuator 200 may be of the vois coil type or similar.
The operation will now be explained with reference to the diagram which shows an example. By detecting the temperature, a change in the relative distance between the semiconductor laser 1 and the collimator lens 2 can be obtained. The actuator 200 is driven to move the collimator lens 2 in the optical axis direction, thereby compensating for the change in temperature. In such cases, however, an adjusting accuracy of about 1 .mu.m is required as the adjusting accuracy of the collimator lens 2 mentioned above. Consequently, a servo operation may be made inoperative by disturbances such as vibration or shock when the collimator lens 2 is likely to move by a distance on the order of 1 .mu.m or more which will lead to practical problems.
Particularly in the case of optical disks, there is a possibility that the whole optical path system will be moved at high speed in the radial direction of the information carrier when information is retrieved. There is, therefore, always a possibility of disturbance such as vibration occurring.